The present invention relates to a device for inspecting a material provided with a coated surface, preferably glass, and a related method.
Surface coatings on materials, in particular glass, are intended to improve or modify the properties of these materials, for example, as antireflective coatings or thermal barrier coatings. Surface coatings comprise thin metal layers or dielectric layers, for example. Coated surfaces, in particular those having a plurality of layers, form interferences (interference layers), which cause the surfaces to exhibit different colors from different viewing angles. These colors are predefined by the thickness of the interference layers.
For large glass surfaces, e.g., glass facades, such color effects can function as a design. In this connection, the objective (possibly in addition to other functions of these layers) is to have the entire glass facade appear homogeneous in a single, predefined color (often green or blue), wherein deviations from these colors are perceived to be unattractive. This means that the light reflected by the material surface has a certain color, i.e., a certain spectrum of wavelengths. For large surfaces of this type, it is furthermore desirable that the facade color not change, even when the viewing angle changes, for example when the observer passes by this facade.
When such materials having coated surfaces are produced, defects can occur that cause the color of the surface to no longer appear homogeneous. A distinction is made in this case to two different types of defects, namely defect type 1, in which the color changes when viewed from a constant viewing angle and defect type 2, which results when the surface has the same color under a constant viewing angle, but the color of the surface changes when the viewing angle changes. This effect always occurs, in principle, with multiple coatings, although the goal is to minimize this defect type to the greatest extent possible by maintaining certain layer thicknesses. The reason for the differentiation into the two aforementioned defect types is that the two defect types have different causes and require different countermeasures in an effort to prevent their occurrence. Therefore, it is also important to know which defect type is present when a color deviation occurs.
Up to now, the inspection of the surfaces was carried out using a plurality of spectrometers, which determine the color of the light reflected on the surface at at least two viewing angles. Since the color of only a small region of the surface can be determined in each spectrometer, the surface must be scanned in a pointwise manner using these spectrometers. This is highly complex and time-consuming, however, and it would be desirable to find a simpler and more cost-favorable solution for inspecting the color of coated surfaces.
In addition to the above-described color defects, it also is possible for a plurality of other defects to occur during the production of such surfaces that do not affect the color, such as optical defects, bubbles, inclusions and scratches. Nowadays, high-quality surfaces, for example, glass that is used to build glass facades, are nearly always subjected to an optical inspection for these other defects. These inspection systems operate with high resolution since the defects being sought are small. The color information obtained from the reflected light is not required for the investigation of such defects, and therefore the optical recording devices used to detect such defects do not capture the color information.